Blasting operations frequently trigger a series of explosions in an exact order, with precise timing. For this purpose, blasting systems have been developed that employ shock tubes (also known as signal transmission lines) that transfer a blast initiation signal to an explosive charge. A signal from a single shock tube can be transferred to multiple shock tubes in a blasting system via the use of connector block/detonator assemblies, thereby permitting the initiation of multiple explosive charges in a controlled manner.
Safety and reliability are paramount for any blasting system, and efficient shock tube initiation is an important factor in this regard. Shock tube initiation failure results in unexploded charges at the blast site, with inevitable safety concerns. Moreover, the reliable initiation of shock tubes ensures that the required blasting pattern is effected.
The efficiency of shock tube initiation depends primarily upon connector block design. Reliable initiation of shock tubes requires sufficient energy to be transferred from the base charge of the detonator to the shock tubes, thereby compressing the shock tubes extremely rapidly to initiate them.
The shock tube retention means of a connector block holds one or more shock tubes in contact with, or close proximity to, the percussion-actuation end of the detonator retained within the block. Importantly, the shock tube retention means ensures that the shock tubes are retained in signal transmission relationship with the detonator. Several examples of connector block designs are known in the art, which comprise a shock tube retention means for the arrangement of at least one shock tube adjacent to the percussion-actuation end of the detonator. These examples generally encompass the use of a clip-like member, integral with the connector block, for retaining the shock tubes within a slot formed between the clip-like member and the percussion-actuation end of the detonator. In this way, the shock tubes are retained in signal transmission relationship with the end of the detonator.
In one example, U.S. Pat. No. 5,204,492, issued to ICI Explosives USA Inc. on Apr. 20, 1993, discloses a detonator assembly for initiating up to eight transmission lines. The assembly comprises a connector block that houses a low strength detonator. The connector block comprises a confining wall surrounding the closed end of the low strength detonator. One or more signal transmission lines can be inserted through a gap in the confining wall and operatively confined adjacent to the percussion-actuation (closed) end of the low strength detonator.
Corresponding U.S. Pat. Nos. 5,171,935 and 5,398,611, issued to the Ensign Bickford Company on Dec. 15, 1992 and Mar. 21, 1995 respectively, disclose a connector block having a housing with a channel formed therein for receiving a low energy detonator. The connector block further comprises a tube engaging member for holding transmission tubes adjacent an end of the channel, wherein the tube engaging member is attached to the connector block via a resiliently deformable segment. Transmission tubes may be inserted into a slot formed between the housing and the tube engaging member.
In another example, U.S. Pat. No. 5,703,319, issued to the Ensign Bickford Company on Dec. 30, 1997, discloses a connector block comprising a clip member. The clip member cooperates with the signal transmission end of a body member to define a slot for receiving one or more signal transmission lines in communication with the output end of a detonator. The clip member is characterized in that it comprises a section of continuously decreasing thickness to facilitate lateral insertion of signal transmission lines into the slot by deformation of the clip member.
U.S. Pat. No. 5,499,581 issued to the Ensign-Bickford Company on Mar. 19, 1996, discloses a connector block design for connecting signal transmission lines in a blasting system. The patent discloses improved means for securing a detonator within the connector block via a displaceable locking member. The connector block may further comprise a flexible, cantilevered line retaining means to receive one or a plurality of outgoing signal transmission lines.
In a final example, U.S. Pat. No. 5,659,149 issued to the Ensign-Bickford Company on Aug. 19, 1997, discloses a connector block including a slot configured to constrain just a single acceptor line retained therein in an undulate configuration i.e. a configuration having consecutive (multiple) bends or kinks including zig-zags. In this way the acceptor line is retained more securely within the slot by virtue of the multiple contortions introduced into the acceptor line, thereby preventing unwanted sliding or displacement of the connector block along the acceptor line.
The connector blocks disclosed by the prior art generally retain at least one shock tube in signal transmission relationship with the percussion-actuation end of a detonator by confining the shock tube(s) within a slot. Preferably, the slot is dimensioned to retain the shock tubes in signal transmission relationship with the detonator, without unduly squeezing the shock tubes. In this way, the energy of detonator actuation compresses the shock tubes extremely rapidly, thereby resulting in their initiation.
The inventor of the present application has determined that optimal energy transfer requires contact between the shock tubes and the surface of the percussion-actuation end of the detonator (or the surface of a positioning surface, which is in contact with the detonator). However, the inventor has noted that dimensional tolerances in the manufacture of connector blocks and shock tubes can result in poor shock tube/detonator contact. For this reason, the insertion of an undersized shock tube into an oversized slot of a connector block can result in poor shock tube/detonator contact, and reduced transfer efficiency of actuation energy. Therefore, manufacturing tolerances can contribute significantly to shock tube initiation reliability. Furthermore, an undersized shock tube in an oversized slot would allow the block to slide uncontrollably to other undesirable locations along the shock tube.
In addition, plastic connector blocks comprising flexible shock tube retention means can exhibit variations in slot dimensions. The connector blocks of the prior art generally comprise flexible and resilient shock tube retention means in the form of a clip, for holding the shock tubes in signal transmission relationship with the percussion-actuation end of a detonator. The flexibility of the shock tube retention means can permit facile shock tube insertion. However, the inventor of the present application has determined that shock tube retention means of this kind may not properly reassume their original shape after distortion, thereby affecting the width of the shock tube retention slot. Moreover, the presence of one or more shock tubes within the slot can alter the configuration of the shock tube retention means, thereby affecting slot width for subsequent shock tube insertion. These factors may further increase the risk of improper shock tube/detonator contact within the connector block.
Accordingly, there is a need for improved connector block designs, wherein shock tubes are positioned in efficient signal transmission relationship with the percussion-actuation end of a detonator, and preferably in material contact with the detonator.